<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
</head>
<body>
	<script type="text/javascript">
		const characters = [
		  'ironman',
		  'black_widow',
		  'hulk',
		  'captain_america',
		  'hulk',
		  'thor',
		];

		console.log(characters.indexOf('hulk'));   	// 2
		console.log(characters.indexOf('batman')); 	// -1

		console.log(characters.includes('hulk'));   // true
		console.log(characters.includes('batman')); // false
	</script>
</body>
</html>